Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as an alternative energy for replacing fossil fuels. In a solar cell, carriers (electrons and holes) generated by light irradiation to a photoelectric conversion section composed of a semiconductor junction or the like are extracted to an external circuit to generate electricity.
Crystalline silicon-based solar cells using a single-crystalline silicon substrate or a polycrystalline silicon substrate are receiving more attention as solar cells with high conversion efficiency. In one form of the crystalline silicon-based solar cell, a photoelectric conversion section including a semiconductor junction is formed by diffusing conductive impurities such as phosphorus atoms to the light-receiving side of a crystalline silicon substrate of a first conductivity-type (p-type) to form a silicon layer of an opposite conductivity-type (n-type) (hereinafter, such a form may be referred to as a “pn-junction crystalline silicon-based solar cell”).
As one form of the crystalline silicon-based solar cell, a solar cell with a semiconductor junction formed by forming an amorphous silicon-based thin-film on a surface of a crystalline silicon substrate is also known. In such a crystalline silicon-based solar cell including an amorphous silicon-based thin-film, the photoelectric conversion section includes an amorphous silicon-based thin-film of an opposite conductivity-type (p-type or n-type) and a transparent electrode layer on the light-receiving side of the crystalline silicon substrate of a first conductivity-type (n-type or p-type), and an amorphous silicon-based thin-film of the first conductivity-type and a transparent electrode layer on the back side of the crystalline silicon substrate (hereinafter, such a form may be referred to as a “heterojunction solar cell”).
In these crystalline silicon-based solar cells, metal collecting electrodes are provided on a light-receiving surface side of the photoelectric conversion section for efficiently collecting carriers generated at the photoelectric conversion section to the external circuit. The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. This method is simple in terms of the process itself, but has such a problem that the material cost of silver is high, and the resistivity of the collecting electrode increases when a silver paste material containing a resin is used. For decreasing the resistivity of the collecting electrode formed of a silver paste, it is necessary to thickly print the silver paste. However, since the line width of the electrode increases with the increase of the print thickness, thinning of the electrode is difficult, and the shading loss by the collecting electrode increases.
For solving these problems, a method is known in which a collecting electrode is formed by a plating method excellent in terms of material and process costs. For example, Patent Documents 1 and 2 disclose a formation method in which a metal layer made of copper or the like is formed by a plating method on a transparent electrode that forms a photoelectric conversion section. In this method, first, a resist material layer (insulating layer) having an opening section matching the shape of a collecting electrode is formed on the transparent electrode layer of the photoelectric conversion section, and a metal layer is formed at the resist opening section of the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a predetermined shape.
Patent Document 3 discloses a method in which an insulating layer of SiO2 or the like is provided on a transparent electrode, a groove extending through the insulating layer is then provided to expose the surface or side surface of the transparent electrode layer, and a metal collecting electrode is formed so as to be in conduction with an exposed area of the transparent electrode. Specifically, a method is proposed in which a metal seed is formed on the exposed area of the transparent electrode layer by a light-induced plating method or the like, and a metal electrode is formed by electroplating with the metal seed as an origination point.
Patent Document 4 proposes a method in which a discontinuous insulating layer having an opening is formed on an electroconductive seed including metal particles, and a metal electrode is formed via the opening of the insulating layer by electroplating.
The crystalline silicon-based solar cell is known to have the problem of leakage due to a short-circuit between the light-receiving side and the back side of the crystalline silicon substrate. For example, the pn-junction crystalline silicon-based solar cell has the problem that in formation of a silicon layer of an opposite conductivity-type (e.g., n-type) on a surface of a crystalline silicon substrate of a first conductivity-type (e.g., p-type) on the light-receiving side, conductive impurities such as phosphorus atoms diffuse to the side surface and the back side of the crystalline silicon substrate, leading to a short-circuit between the light-receiving side and the back side. The heterojunction solar cell has the problem that in formation of thin-films such as an amorphous silicon layer and a transparent electrode layer on a crystalline silicon substrate by a plasma-enhanced chemical vapor deposition (CVD) method, a sputtering method or the like, the thin-films are not only formed on a principal forming surface of the silicon substrate, but also formed so as to wrap around to the side surface or the vicinity of the peripheral end of a principal surface on a side opposite to the principal forming surface, resulting in occurrence of a short-circuit between electrodes on the surface and the back surface and between amorphous silicon layers.
For eliminating such a short-circuit between the front and the back surfaces of the crystalline silicon substrate, a method has been proposed in which an insulation process is performed by laser processing. For example, Patent Document 5 discloses a method in which a silicon layer of an opposite conductivity-type is formed on the light-receiving side of a crystalline silicon substrate of a first conductivity-type, and an end of the crystalline silicon substrate on the back side is subjected to laser processing to form a groove, so that the silicon layer of the opposite conductivity-type is removed. Patent Document 6 discloses a method in which a groove is formed by laser processing to remove a conductive silicon-based thin-film and a transparent electrode layer formed on a surface of a crystalline silicon substrate, so that a short-circuit between the front and the back surfaces is eliminated.
Patent Document 6 illustrates removal of the transparent electrode layer and the conductive semiconductor layer by applying laser light, but it is difficult to selectively remove these layers by applying laser light. Thus, a groove formed by laser processing generally extends to a surface or the inside of a crystalline silicon substrate. Patent Document 7 indicates the problem that when laser processing is performed from the side of a surface which is provided with a semiconductor layer of the opposite conductivity-type, a silicon substrate-exposed area such as a groove formed by laser processing, an end surface or the like becomes an additional leak spot, leading to a reduction in open circuit voltage and fill factor. In view of the problem, Patent Document 7 proposes a method in which a groove is formed by laser processing from the side of a surface which is provided with a semiconductor layer of the first conductivity-type having a conductivity type identical to that of a silicon substrate of the first conductivity-type, and the silicon substrate is then cleaved with the groove as an origination point.
Patent Document 8 proposes a method in which, for integration of a thin-film silicon-based solar cell, a groove for removing a back electrode layer is formed by laser processing to insulate adjacent cells from each other, and the processed portion is heated to perform insulation although the method is not related to a crystalline silicon-based solar cell. According to this method, a leak spot generated due to laser processing is insulated, and therefore a leakage current is reduced, so that the fill factor of an integrated thin-film solar cell can be improved.